The prior art has concerned itself, for many years, with the provision of porous electrodes that are particularly intended for use in fuel cells. However, while many quite acceptable electrodes have been provided in the prior art, it has been the general experience that such electrodes are expensive to produce. This comes, especially, due to the generally accepted requirement for the use of noble metals in fuel cell electrodes, including especially gas diffusion or porous electrodes. Moreover, when the prior art has provided electrodes which comprise a porous substrate having a porous catalytically active layer thereon, it has been common in the past for there to be relatively high catalyst loading by weight per unit area of geometrical electrode surface, thereby contributing further to the cost of producing such electrodes.
Such prior art has included KORDESCH et al U.S. Pat. No. 3,405,010 dated Oct. 8, 1968, and KORDESCH U.S. Pat. No. 3,310,434, dated Mar. 21, 1967. The former patent relates to the catalyzing of porous electrodes, using a heavy metal salt, an aluminum salt, and a ruthenium salt. The latter patent is particularly related to the use of noble metals as catalysts on a porous electrode.
Yet another KORDESCH patent relating to the use of wet proofed conductive substrates having an active conductive layer with a surface-deposited noble metal catalyst is U.S. Pat. No. 3,899,354, issued Aug. 12, 1975.
BAKER et al in U.S. Pat. No. 3,935,029 issued Jan. 27, 1976 teach the use of fine graphite particules enmeshed in a web of polytetrafluoroethylene (PTFE), however, once again using noble metals.
Thus, it is a principal purpose of the present invention to provide catalyzed electrodes having excellent performance characteristics, at low cost. The catalyzed electrodes of the present invention are specifically adapted for use in fuel cells and metal-air cells; and especially useful as auxilliary gas recombining electrodes in alkaline zinc/manganese dioxide cells (particularly secondary cells), or other primary or secondary alkaline cells. The porous catalyzed electrodes of the present invention have particular utility as oxygen reduction electrodes in alkaline cells as noted above.
Thus, the present invention comprises the provision of a porous electrode which comprises a porous conductive substrate and a porous catalytically active layer on the porous conductive substrate; the porous conductive substrate being chosen from the group consisting of carbon, graphite, and metal; and the porous catalytically active layer being chosen from the group consisting of a catalytically active non-noble metal, an oxide of a catalytically active non-noble metal, carbon, carbon together with a catalytically active non-noble metal, and carbon together with an oxide of a catalytically active non-noble metal. (If used as an anode in a fuel cell, the electrode of the present invention is as described above, together with a further additional smaller amount of a catalytically active noble metal or carbon together with a catalytically active noble metal.)
Catalytically active non-noble metals that are particularly contemplated for use in the present invention include iron, cobalt, nickel, manganese, chromium, copper, and vanadium; and catalytically active noble metals there particularly intended for use in the present invention, in anodes according to this invention, include platinum, palladium, rhodium, iridium, osmium, gold, silver, and ruthenium.
In general, a porous electrode according to the present invention may include the porous active layer chosen from the group consisting of carbon together with a catalytically active non-noble metal, and carbon together with an oxide of a catalytically active non-noble metal; and very often, the porous active layer may further comprise polytetrafluoroethylene (PTFE), as a binder. The carbon may be graphite.
Generally speaking, fuel cells may be considered to be galvanic cells, with the basic reaction being the electrochemical oxidation of a fuel and the electrochemical reduction of an oxidant (e.g., oxygen). It should be noted, however, that fuel cells differ from ordinary primary cells such as commercially available dry cells, in that the fuel and oxidant are generally introduced continuously into the cell electrodes during the production of electricity. Thus, theoretically, the electrodes and electrolyte of fuel cells should maintain a constant value, during the time when the fuel and the oxidant are reacted electrochemically within the fuel cell, and electricity and the product of reaction--usually water--are removed from the fuel cell.
There has been, for the last century or so, a continuing search for ways to boost the electrical output of fuel cells, and/or to increase their service life, and/or to lower the cost of producing fuel cells so as to render them commercially feasible. Needless to say, one major area for research has been the catalysis of reactions which take place within the electrodes of fuel cells, and thereby the requirement for discovering new methods of depositing known catalysts either in a more active form or more economically. However, the search still continues for catalysts which will raise the current density within electrodes, and/or the voltage of the cell, to levels which approach those that are attainable in theory.
On the other hand, there has also developed a pressing need for gas recombining electrodes in closed cells such as alkaline zinc/manganese dioxide cells--especially secondary cells. In such cells, there may be a periodic (or continuing) generation of gasses, and particularly there may be evolution of gaseous oxygen on charge, overcharge, or any reversal of cell polarity. Such cells typically operate over a broad range of temperatures (e.g. from -40 to +65 degrees celsius) and at current densities where the auxilliary porous electrode may itself be required to pass up to 750 mA/sq. cm.
It is the position of the present inventors that the present invention provides a major step in the required direction, by providing catalyzed porous conductors having relatively low production costs, and with excellent characteristics as to their efficiency, power density, performance, and life. The characteristics of electrodes according to the present invention compare most favourably with the characteristics of certain higher efficiency prior art fuel cell electrodes which, however, contained noble metals in all instances and which therefore were costly to produce.
When used as a catalyzed porous electrodes in fuel cells or alkaline zinc/manganese dioxide cells, electrodes of the present invention may comprise a porous gas diffusion layer adhered to the porous conductive substrate spoken of above, at one side thereof. In fuel cells, electrodes according to the present invention may also generally include porous metal current collectors, as a convenient way of conducting electrical current produced in the fuel cell out of the fuel cell.
In general, the catalyst provided by the present invention is insoluble within the operating voltage and the operating condition range of the electrode, thereby contributing to the life of the electrode. This means that catalyzed porous electrodes according to the present invention may be used with such electrolytes as alkaline electrolytes, for example potassium hydroxide as used in zinc/manganese dioxide cells. It also makes the use of catalyzed porous electrodes according to the present invention in metal-air batteries and cells more attractive.
The inventors herein have discovered, quite unexpectedly, that the addition of a metal or metal oxide from the group consisting of iron, cobalt, nickel, manganese, chromium, copper, and vanadium--all non-noble metals--to a porous electrode provides an electrode having a service life and performance characteristics comparable to those of platinum catalyzed electrodes. Clearly, the cost of electrodes according to the present invention, when compared to the cost of a platinum catalyzed electrode, may be considerably lower. Moreover, platinum catalyzed electrodes may have significant carbon corrosion of the carbon within the porous substrate structure, whereas the use of metals and metal oxides according to the present invention significantly decreases the risk of carbon corrosion. That fact contributes, therefore, to the attractiveness of electrodes according to the present invention for use in alkaline cell systems, and metal-air batteries. In addition, dissolved trace amounts of noble metals can cause problems in metal-air or other batteries containing a zinc, iron or aluminum anode by enhancing the hydrogen evolution (gassing reaction). This enhanced anode corrosion causes higher self-discharge and can result in cell leakage.
According to the present invention, the porous conductive substrate of electrodes may be carbon, graphite, or metal; or, indeed, any other suitable conductive material. The porous active layer may comprise a catalytically active non-noble metal as discussed above, an oxide of a catalytically active non-noble metal, carbon, carbon together with a catalytically active non-noble metal, and carbon together with a oxide of catalytically active non-noble metal. Typically, the porous active layer consists of carbon together with a catalytically active non-noble metal or carbon together with an oxide of catalytically active non-noble metal; and in either case, it may further comprise a binder, usually PTFE.
A typical porous electrode according to the present invention would consist of an electrochemically active layer which may be typically from 50 to 500 microns thick, and it may further include a gas diffusion layer which may also be 50 to 500 microns thick. If used in a fuel cell, the electrode would usually also have a metal screen current collector (a porous metallic current collector) embedded in an electrode layer--unless a bipolar construction method is applied. The electrochemically active layer, as noted would contain the electrocatalyst as described above, supported on a carbon, graphite or metal porous substrate, and is located on the electrolyte side of the electrode.
A gas diffusion layer, when used, may typically consist of PTFE bonded carbon, and would normally have a higher degree of hydrophobicity than the active layer. During the manufacturing process of the gas diffusion layer, discussed in greater detail hereafter, a pore builder such as ammoniumbicarbonate, may be used. The current collector, when used, may be embedded in either the gas diffusion layer or the active layer; and when electrodes according to the present invention are used in bipolar cells, no current collector is required.
Typically, the total electrode thickness of an electrode comprising an active layer, a gas diffusion layer, and a current collector, may range from 100 to 750 microns.
Thus, porous electrodes according to the present invention may comprise a porous conductive substrate, and a porous gas diffusion layer adhered to one side thereof. As noted, the porous active layer is at a first side of the porous conductive substrate, and the porous gas diffusion layer is at the side of the porous conductive substrate opposite to that of the active layer. Of course, the porous active layer may permeate the porous conductive substrate, and in any event the porous gas diffusion layer is adhered to one side thereof.
As noted above, electrodes according to the present invention may also be used as anodes in fuel cells, in which case a further additional smaller amount of a catalytically active noble metal or carbon together with a catalytically active noble metal is added to the catalytically active layer of the electrode. There is little point in providing oxides of catalytically active noble metals, since the oxide would be promptly reduced to the noble metal per se when exposed to the hydrogen fuel generally used in a fuel cell.
The noble metals contemplated for use in this aspect of the invention are discussed above.
The concentration of catalyst within the porous catalytically active layer is generally in the range of from 0.1 to 10 mg per square centimeter of the geometrical electrode surface area. Typically, the concentration of catalyst within the porous catalytically active layer is in the range of from 1 to 5 mg per square centimeter of the geometrical electrode surface area.
In general, cathodes for hydrogen/oxygen fuel cells, according to the present invention, may have a service life well in excess of two thousand hours at a current density of 100 milliamperes per square centimeter, at a voltage above 0.85 volts versus RHF (Reversible Hydrogen Electrode). Similar electrodes, using non-noble metal oxide catalysts, can be run at up to 300 miliamperes per square centimeter on oxygen and air, with potentials of about 0.8 volts and 0.79 volts versus RHF, respectively. As auxilliary gas recombining electrodes used in alkaline electrochemical cells such as zinc/manganese dioxide cells having potassium hydroxide electrolytes, the electrodes may be successfully subjected to current densities up to 750 mA/sq. cm. at low temperatures.
The present invention provides three generally related but distinct processes for the production of porous electrodes in keeping herewith. Those three general methods may be characterized as follows:
I PA1 II PA1 III
(a) impregnating a porous conductive substrate structure with a compound containing the chosen material for the porous catalytically active layer; and PA2 (b) forming the porous catalytically active layer: PA2 (a) mixing the chosen material for said porous catalytically active layer with the chosen material for the porous conductive substrate; and PA2 (b) fabricating the electrode: PA2 (a) depositing pyrolitic carbon from the gas phase onto a porous conductive substrate structure, at an elevated temperature in a gas atmosphere.
The compound used in step (a) of Process I, above, is a metal salt solution of the chosen catalytically active metal. The formation process of step (b) of the Process I may be the chemical formation of the porous active layer, or the thermal formation of the porous active layer.
The chosen active material that is used in step (a) of the Process II, may further be mixed with carbon and PTFE.
The Process III is carried out at least 600 degrees C, and the gas atmosphere may be steam, carbon dioxide, carbon monoxide, ammonia, nitrogen, argon, or hydrogen.